This invention relates to wavelength locking of lasers and optical test instruments.
Optical fiber communication systems provide for low loss and very high information-carrying capacity. In practice, the bandwidth of optical fiber may be utilized by transmitting many distinct channels simultaneously using different carrier wavelengths. The associated technology is called wavelength division multiplexing (xe2x80x9cWDMxe2x80x9d). In a narrow bank WDM system, 50 or more different wavelengths are closely spaced to increase fiber transmission capacity.
The wavelength bandwidth that any individual channel occupies depends on a number of factors, including the impressed information bandwidth, and margins to accommodate for carrier frequency drift, carrier frequency uncertainty, and to reduce possible inter-channel cross-talk due to non-ideal filters.
To maximize the number of channels, lasers with stable and precise wavelength control are required to provide narrowly spaced, multiple wavelengths.
Some laser sources, for example vertical cavity surface emitting lasers (xe2x80x9cVCSEL""sxe2x80x9d), exhibit wavelength drift over time, in excess of the requirements for narrow band WDM applications. More particularly, the wavelength of the device tends to change with aging under continuous power. Since telecommunication systems are expected to have a lifetime on the order of 25 years or so, wavelength control must be added to the laser transmitter to ensure minimum cross-talk between narrowly spaced channels over extended time periods.
Single wavelength optical communications systems are widely used in the industry. Ideally, systems designers seek minimum disruption of existing systems, and compatibility with existing packaging, in the development of WDM systems.
Typically, known laser wavelength monitoring and stabilization systems are based on a unit external to the standard package of a laser source (or xe2x80x9ctransmitterxe2x80x9d). One commercially available system for monitoring and control of the wavelength of a semiconductor laser is an assembly based on crystal gratings. For example, in a known system manufactured by Accuwave, and described in the product literature, a wavelength locker unit is provided which comprises a lithium niobate crystal in which two Bragg gratings are written, illuminated by a collimated beam from a laser source coupled to the assembly, and two photodetectors. Each grating has a slightly different Bragg wavelength and angle relative to the input beam. The output reflected from the gratings is directed to the two detectors and the differential output is used to provide feedback control to the laser. Wavelength stability of better than 10 pm can be achieved with the control loop. However, the wavelength locker unit utilizes a separate unit from the transmitter, and thus requires external coupling to the laser or light source. Moreover, the unit is designed for a specific wavelength, as specified by the grating parameters. Different units are required for different wavelengths.
Another known type of wavelength monitoring/control assembly is based on a fiber grating. For example, GB Patent Application No. 96/00478, filed Mar. 4, 1996 by Epworth et al., relates to an external cavity type laser whose external reflector is provided by a Bragg reflector located in an optical fiber butted to an anti-reflection coated facet of the semiconductor laser. The grating is placed far enough from the laser that the longitudinal modes are so closely spaced that the laser operates multimode with so many modes as to make mode partition noise negligible. Another GB Patent Application No. 95/19614.3, filed Sep. 26, 1995 by Epworth et al., relates to using a chirped fiber grating for equalization and laser frequency stabilization.
Fabrication of fiber grating assemblies is complex. As with the crystal grating system mentioned above, fiber gratings are fabricated to match the specific wavelength of the transmitter, and the assembly is therefore wavelength specific.
Another system for stabilization of a semiconductor laser is described in U.S. Pat. No. 4,309,671 to Malyon which uses a pair of matched photodiodes and two beam splitters. The first beam splitter and first photodiode monitor power, and a second beam splitter, a frequency dependent filter and second photodiode are used to monitor wavelength changes. The outputs of the matched photodiodes are fed via amplifiers to a subtractor amplifier and the output is fed as negative feedback to the amplifier controlling operation of the laser.
Other known systems are based on a filter element such as a Fabry-Perot etalon. For example, U.S. Pat. No. 5,331,651 to Becker et al. describes the use of a Fabry-Perot etalon for fine tuning in conjunction with a grating for coarse tuning of the output of a laser.
In a system described in U.S. Pat. No. 5,438,579 to Eda et al., a Fabry-Perot etalon is used with a single photodetector to generate a signal used to lock onto one peak of a semiconductor laser, requiring collimated beams. Hill et al., in U.S. Pat. No. 4,839,614, describe a system for referencing frequencies of radiation from multiple sources relative to a reference source, using a filter element such as a Fabry-Perot etalon and a corresponding plurality of detectors.
Another system for laser wavelength stabilization is described in U.S. Pat. No 4,914,662 to Nakatani et al. which involves spectroscopically processing the output of a variable wavelength laser and measuring a spatial distribution using image processing apparatus, and then comparing the distribution to that of a reference light source of fixed wavelength. The latter image processing system is complex, and not readily compatible with providing a low cost, compact unit.
Japanese Patent Application No. 92-157780 relates to a frequency stabilizer for a semiconductor laser, without using external modulating means, and is based on an inclined Fabry-Perot etalon on which the laser source is incident, and two photodetectors to detect, respectively, the transmitted and reflected signals. By subtracting outputs of the two detectors, a signal is provided for controlling the oscillation frequency. Resonator length is altered by changing the inclination of the etalon to allow for tunability. The implementation of this system for minimum space requires using the F-P (i.e., the Fabry-Perot etalon) at a relatively large angle, with decreased stability in terms of center wavelength and bandwidth. On the other hand, a small F-P angle requires added components and space, as shown in FIG. 1B of the aforementioned Japanese patent application. Also, independent detectors are used, with potentially different response and aging characteristics.
The system described in U.S. Pat. No. 5,825,792 to Villeneuve et al. uses a differential technique along with a Fabry-Perot etalon to stabilize a laser. The system of U.S. Pat. No. 5,825,792 has some similarities to the present invention; however, the system of U.S. Pat. No. 5,825,792 does not provide an independent absolute wavelength determination capability and could be difficult to implement in WDM systems with a very large number of channels, such as 40 or 80 channels.
Consequently, various existing systems for wavelength stabilization are known using a crystal grating, fiber grating or etalon based arrangement. The grating based systems lack wavelength tunability, and many systems are based on relatively large control units external to a packaged laser source with concurrent coupling, space and power dissipation problems. While etalon based systems provide tunability, none of the known configurations are sufficiently compact to incorporate in known standard packages without disruption.
The primary object of this invention is to provide a temperature stable wavelength reference device in a low cost compact device. Multiple wavelength (or, alternatively, frequency) references, located at previously defined absolute locations, are generated. Two signals generated simultaneously by passing collimated light through a single etalon at two properly chosen, distinct angles provides the information needed for absolute frequency determination at any one of a large number of frequencies spaced on an evenly spaced frequency (i.e., ITU) grid.
The present invention provides a compact wavelength monitoring and control assembly, preferably for integration within a small semiconductor laser package and for application in WDM optical transmission systems.
Thus, according to one aspect of the present invention, there is provided a wavelength monitoring and control assembly for an optical system comprising a divergent laser emission source, the assembly comprising;
first and second photodetectors spaced apart by a specific separation, and located at a specific distance from the emission source;
a narrow bandpass wavelength selective transmission filter element, of Fabry-Perot structure, located between the source and the detectors;
a phase grating plate, to split an incident collimated optical beam into two or more beams with a predetermined angular relationship, located between the filter element and the source;
a collimating lens located between the phase grating and the source; and
a control loop for feedback of a control signal, which is generated as a function of the difference and ratio of the signals generated by the first and second photodetectors in response to a change in wavelength of the emission source, to a control means of the emission source; a table stored in the controller contains the information required to select the required ratio after the target wavelength is known.
Optionally, an external computer system (e.g. a personal computer or systems control device) may be used to select or change the desired wavelength.
Thus, a simple and compact wavelength monitoring and control assembly with an internal absolute wavelength reference for a laser emission source is provided. Because the transmission of a Fabry-Perot filter is characterized by a series of transmissive peaks at regular frequency intervals, for example, at 100 GHz spacing, simultaneous stabilization points are attainable for a plurality of predetermined wavelengths which are determined by the wavelength spacings on the multiple transmissive peaks of the Fabry-Perot filter. The photodetectors are illuminated by beams passing through a narrow bandpass filter at different angles of optical incidence. Thus, wavelength variation of the laser emission source is converted into different photocurrent changes in the two photodetectors. The wavelength variation of the ratio of the two photocurrents is used to identify the particular transmissive peak, and thus the approximate absolute frequency, of the Fabry-Perot filter. The wavelength variation of the difference of the two photocurrents is used simultaneously in a feedback loop to stabilize the wavelength of the source to a desired target wavelength, i.e. through a signal sent back to the laser (transmitter), e.g. via a wavelength tuning voltage, or active area temperature, or current changes, to correct for wavelength drift.
This assembly allows for precise optical monitoring of the wavelength to provide a control signal for wavelength stabilization, to maintain the laser wavelength within the limits required to reduce cross-talk for use in, for example, a WDM optical transmission system. A difference signal is advantageous also to provide immunity to fluctuations in source output power.
The narrow bandpass wavelength selective transmission filter element is a Fabry-Perot structure. The photodetectors are preferably a matched pair of photodiodes. Through the angular dependence of the wavelength transmission of the Fabry-Perot etalon, a wavelength variation from the source is converted to a transmission loss, and the wavelength change is detected as a power change. Thus, the device functions as an optical wavelength discriminator in which the detector converts optical energy to current for a feedback loop for controlling the light source. For determination of the approximate absolute wavelength, the ratio of the two photocurrents is used to identify the Fabry-Perot order. For wavelength stabilization, the differential output of the two photodetectors is used in a feedback loop to stabilize the wavelength of the laser source to a desired target wavelength.
Beneficially, the angle of inclination of the filter is adjustable to provide tunability of the predetermined wavelengths. Since the wavelength selective element is a Fabry-Perot etalon, whose transmission characteristics are dependant on the angle of the etalon relative to the beam, the assembly provides for the tunability needed to remove the effects of fabrication and assembly tolerances by adjusting the angle of the etalon.
The assembly is simple to manufacture relative to fabrication of fiber grating systems for wavelength stabilization. This approach provides a dither free discrimination scheme, which avoids frequency modulation and demodulation steps.
Advantageously, the photodetectors are a matched pair of photodiodes. When the gain of each of the two photodetectors is independently adjustable, the predetermined wavelengths may be selected by setting the unequal gain for the two photodetectors.
Optionally, a lens is disposed between the transmissive filter element and the photodetectors to a maximize the power falling on the two detectors and to optimize the separation of the two optical beams. A larger beam diameter passing through the transmissive filter element is preferable to provide a more nearly ideal filter shape to obtain more optimum wavelength selective performance.
The laser emissive source may be an output from a VCSEL, an output facet from a semiconductor laser, or alternatively a cleaved or tapered single mode fiber.
Advantageously, when the laser emission source comprises a laser (semiconductor or VCSEL) provided within a package, the wavelength monitoring assembly is provided within the same package to provide an integral unit. While use of the assembly as an external reference unit is feasible, polarization maintaining fibers and couplers are ideally required to avoid polarization dependence.
Because the monitoring assembly is simple and compact, an important advantage is that the assembly may be co-packaged with the laser source in a transmitter module. Additionally, a single such module may be used as a source at any one of a large number of predetermined wavelengths. This is particularly useful in adapting existing transmitter modules, as used for multi-wavelength transmission systems, for use with additional components for WDM without taking up additional space and with minimum disruption of existing systems.